1. Field of the Invention
The present invention relates to a tandem image forming device including a plurality of image forming means, each of which forms a toner image on a respective image carrier, arranged side by side along an intermediate image transfer body implemented as a belt or along a path for conveying a paper sheet or similar recording medium. Also, the present invention relates to a copier, printer, facsimile apparatus or similar color image forming apparatus including a tandem image forming device.
2. Description of the Background Art
A tandem image forming device includes a plurality of image forming means arranged side by side and each including a charger, an exposing device, a developing device and a cleaning device arranged around an image carrier. Japanese Patent Laid-open Publication No. 9-34205, for example, discloses a color image forming apparatus in which a plurality of image forming means are simply arranged side by side along a path for conveying a paper sheet, OHP (overHead Projector) film or similar recording medium. Each image forming means electrophotographically forms a toner image in a particular color. Such toner images are sequentially transferred to, e.g., a paper sheet one above the other, completing a composite color image. This kind of apparatus, however, has a problem that the distance between nearby image forming means and therefore the overall size of the apparatus increases.
In light of the above, Japanese Patent Laid-Open Publication No. 9-160471, for example, teaches a tandem image forming device in which one of nearby image forming means has its developing device positioned above a cleaning device included in the other image forming device. Specifically, a developing device included in each image forming device uses a toner and carrier mixture, i.e., a two-ingredient type developer and is made up of an agitating section and a developing section. The agitating section conveys the developer while agitating the developer to thereby deposit the developer on a sleeve for development. The developing section transfers the toner of the developer from the sleeve to an image carrier. The agitating section is positioned at a higher level than the developing section. The cleaning device of the image forming means next to the above developing means is positioned below the above agitating section.
The tandem image forming device taught in the above Laid-Open Publication No. 9-160471 successfully reduces the distance between nearby image forming means and is therefore small size. Such an image forming device reduces the overall size of the image forming apparatus. However, the developer fed from the agitating section to the developing section, which is lower in level than the agitating section, accumulates or the sleeve. Moreover, after development, the developer accumulated on the sleeve must be returned to the agitating section against gravity and therefore cannot be smoothly circulated or uniformly mixed in the agitating section. Consequently, when the same image pattern is repeatedly output, the consumption of the toner differs from one position to another position, resulting in irregular image density.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-269641 and 2000-235311.